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# Abstract
object_of_studyresearch_questionconcepts↗concept_edgesThe land, water, and energy requirements of hydroponics were compared to those of conventional agriculture by example of lettuce production in Yuma, Arizona, USA. method↗controlsconcepts↗sensesData were obtained from crop budgets and governmental agricultural statistics, and contrasted with theoretical data for hydroponic lettuce production derived by using engineering equations populated with literature values. controlscontent_tagsYields of lettuce per greenhouse unit (815 m2) of 41 ± 6.1 kg/m2/y had water and energy demands of 20 ± 3.8 L/kg/y and 90,000 ± 11,000 kJ/kg/y (±standard deviation), respectively. content_tagsIn comparison, conventional production yielded 3.9 ± 0.21 kg/m2/y of produce, with water and energy demands of 250 ± 25 L/kg/y and 1100 ± 75 kJ/kg/y, respectively. claim_typemechanisms↗content_tagsHydroponics offered 11 ± 1.7 times higher yields but required 82 ± 11 times more energy compared to conventionally produced lettuce. contributionTo the authors’ knowledge, this is the first quantitative comparison of conventional and hydroponic produce production by example of lettuce grown in the southwestern United States. concepts↗content_tagsIt identified energy availability as a major factor in assessing the sustainability of hydroponics, and it points to water-scarce settings offering an abundance of renewable energy (e.g., from solar, geothermal, or wind power) as particularly attractive regions for hydroponic agriculture.
Keywords:
agriculture; hydroponics; sustainability; water; energy; land use; lettuce; Arizona
<strong>Graphical Abstract</strong>
Graphical Abstract
1. Introduction
Conventional agricultural practices can cause a wide range of negative impacts on the environment. sensescontent_tags↗“Conventional” or “modern industrial agriculture” has been historically defined as the practice of growing crops in soil, in the open air, with irrigation, and the active application of nutrients, pesticides, and herbicides. concepts↗Some of the negative impacts of conventional agriculture include the high and inefficient use of water, large land requirements, high concentrations of nutrients and pesticides in runoff, and soil degradation accompanied by erosion [1, 2]. As the world population continues to grow at a rapid rate, so too must the food production. phenomenoncontent_tagsHowever, approximately 38.6% of the ice-free land and 70% of withdrawn freshwater is already devoted to agriculture [3, 4]. To sustainably feed the world’s growing population, methods for growing food have to evolve.
The benefits of hydroponic agriculture are numerous. In addition to higher yields and water efficiency, when practiced in a controlled environment, hydroponic systems can be designed to support continuous production throughout the year [5]. Hydroponic systems are very versatile and can range from rudimentary backyard setups to highly sophisticated commercial enterprises. Various commercial and specialty crops can be grown using hydroponics including tomatoes, cucumbers, peppers, eggplants, strawberries, and many more. Leafy vegetables, such as lettuce can also be grown hydroponically and perform best using the nutrient film technique (NFT) [6]. Hydroponic NFT production involves the circulation of a nutrient solution through shallow channels in a closed-loop system [7].
In 2012, in terms of production by weight, head lettuce was the second largest vegetable crop in the Unites States, second only to onions [8]. A substantial portion of that production (approximately 29% in 2012) occurs in Arizona, primarily in Yuma [8, 9]. Since Arizona devotes approximately 69% of its current freshwater withdrawals to agriculture [10], investigations into hydroponic alternatives could be beneficial in reducing the strain on water resources in such regions. There is considerable research available regarding conventional lettuce production and hydroponic lettuce production separately, but few studies that have compared the resource inputs of the two at a commercial level.
Regarding conventional lettuce production, in 2001, the University of Arizona Cooperative Extension developed county-specific crop budgets estimating the operating and ownership costs of producing vegetables in Arizona using representative cropping operations and such resource inputs as water, fuel, and fertilizer [11]. Realizing that the water and energy use for agriculture is substantial, Ackers et al. (2008) [12] performed an “order of magnitude” study and determined a reasonable range of estimates for resources used in the production of Arizona agriculture.
Regarding hydroponics, the Ohio State University developed an enterprise model designed to estimate the revenue, expenses, and profitability associated with a typical hydroponic greenhouse lettuce production system in Ohio [13]. Various other authors have investigated components of hydroponic lettuce production as it relates to water and energy inputs [14, 15].
The objective of this study is to determine whether hydroponic lettuce production is a suitable and more sustainable alternative to conventional lettuce production in Arizona. For this study, “a suitable and more sustainable alternative” is one that outperforms (i.e., is more efficient than) conventional agriculture in the metrics of land use, water use, and energy use, normalized by yield.
2. Experimental Section
Metrics for land, water, and energy use were developed for conventional lettuce production and hydroponic lettuce production using a southwestern Arizona location as a case study scenario. Data related to conventional lettuce production was obtained from crop budgets and governmental agricultural statistics, while data regarding the hydroponic production of lettuce was developed using engineering equations and values reported in literature which were then applied to a hypothetical 815 square meter enclosed hydroponic greenhouse located in Yuma, Arizona. This greenhouse was assumed to be using the nutrient film technique of growing hydroponic lettuce, have temperature controls, supplemental artificial lighting, and water circulation pumps.
2.1. Lettuce Grown Using Conventional Agricultural Methods
2.1.1. Conventional Yield
The average yield of head lettuce per acre in Arizona was quantified by taking the average of the past ten years of agricultural census data (2003–2012) that was available from the National Agricultural Statistics Service (NASS) for the United States Department of Agriculture (USDA) [8, 16, 17, 18, 19]. The resulting value was then converted into units of kilograms per square meter per year (kg/m2/y).
2.1.2. Conventional Water Usage
The water usage related to the conventional production of head lettuce in Arizona was estimated using county-specific crop budgets developed by the University of Arizona Cooperative Extension. Specifically, the two Yuma County crop budgets for head lettuce were used to produce an average estimate of irrigation use of 965 L per square meter per crop [11]. The average value was then converted to liters per kilogram per year (L/kg/y) using the estimated average yield value for conventional lettuce and assuming one crop of lettuce per year.
2.1.3. Conventional Energy Usage
Energy use related to the conventional production of head lettuce in Arizona was limited to the direct use of fossils fuels by farms during operations and the use of electricity in the pumping of irrigation water. An average estimate of fuel use was derived from the Yuma County crop budgets for head lettuce production developed by the University of Arizona Cooperative Extension [11]. The average value was then converted to kilojoules per kilogram per year (kJ/kg/y) using the estimated average yield value for conventional lettuce and assuming one crop of lettuce per year. It was conservatively assumed that all farms require energy to pump irrigation water and that all of the pumps use electricity.
Using an estimate of 1037 kilowatt-hours (kWh) per acre foot pumped [12], a conversion factor of 3600 kilojoules per kWh, the average estimate of water use calculated previously, and the estimated average yield value for conventional lettuce, an average estimate of energy use for the pumping of irrigation water was computed in units of kJ/kg/y.
Together, the average estimates of energy use related to fossil fuels and the pumping of irrigation water were combined to produce an overall average estimate of energy use for the conventional production of head lettuce in Arizona in units of kJ/kg/y.
2.2. Lettuce Grown Using Hydroponic Methods
2.2.1. Hydroponic Yield
The average yield of head lettuce grown using hydroponic methods was estimated by evaluating values reported in available literature. Databases searched included; AGRICOLA, Web of Science, Google Scholar, and Science Direct, using keywords “hydroponics” and “head lettuce.” Only publications that contained the necessary information regarding yield, type of hydroponic system (e.g., NFT), configuration, production area, and greenhouse area were used. Once the information was gathered, the data was standardized.
Configurations from three studies were averaged to get an approximate plant density of 24 plants per square meter of greenhouse [7, 13, 20]. With supplemental artificial lighting and temperature controls lettuce production can be continuous year round, with a full harvest cycle completed every 30 days (or approximately 12 harvests per year) [13]. The mass of edible lettuce per plant was averaged from two sources to get an average mass of 144.6 grams per plant [13, 21]. Using the estimates for plant density, number of harvests, and plant mass, the average yield of hydroponic lettuce per year was calculated in units of kg/m2/y.
2.2.2. Hydroponic Water Usage
The water usage related to the hydroponic production of head lettuce in Arizona was estimated by considering the average evapotranspiration of each individual plant, which takes into account the water loss due to evaporation and transpiration. Both (2014) [22] reported that lettuce plants grown hydroponically must transpire at least 400 mL per gram of dry mass accumulated to avoid tipburn damage. Using a ratio of dry mass to fresh mass of 0.045 [22] and the average mass per head of lettuce calculated previously, the average evapotranspiration per plant was calculated. This value was then conservatively increased by an additional 10% to account for periodic flushing of the nutrient solution. This estimate was used in combination with the estimated yield values for hydroponic lettuce to create an average estimate of water usage in units of L/kg/y.
2.2.3. Hydroponic Energy Usage
Energy use related to the hydroponic production of head lettuce in Arizona focused on the energy demands associated with supplemental artificial lighting, water pumps, and heating and cooling loads.
The energy use related to supplemental artificial lighting was determined by assuming a 24-h photoperiod and a recommended daily Photosynthetically Active Radiation (PAR) of 17 mol/m2/day of both natural and supplemental light [5], for optimal lettuce production. It was also assumed that half of the required radiation could be obtained from natural lighting. Assuming a wavelength of 600 nm (the approximate average wavelength of the high pressure sodium bulbs used in supplemental light), the following equation was used to calculate the energy of a mole of photons:
E=h×cλ×6.022×1023mol
(1)
where
- E: Energy per mole of photons (J/mol)
- h: Planck’s constant (6.626 × 10−34 J·s)
- c: Speed of light (2.998 × 108 m/s)
- λ: Wavelength of light (m)
The resulting value was then used in conjunction with the estimated yield values for hydroponic lettuce to calculate the energy demand from supplemental lighting in units of kJ/kg/y.
The energy use related to the pumping of water associated with the hydroponic production of lettuce was estimated using an average pumping time and a representative pump power rating. Lopes da Luz et al. (2008) [15] performed a study indicating that the optimal irrigation regimen for hydroponic lettuce resulted in four hours thirty minutes of pumping per day. Regarding the pump power rating, a study [23] suggested that a 0.5 horsepower (HP) pump is sufficient to deliver water to a hydroponic system with an average of 2040 heads of lettuce. Together, the average horsepower per plant along with the average pumping time per day was used in conjunction with the estimated yield values for hydroponic lettuce to estimate the energy use related to pumping in units of kJ/kg/y.
The energy use related to heating and cooling loads associated with the hydroponic production of lettuce was estimated by calculating the design heat load of a hypothetical greenhouse in Arizona. The following equation, based on standard heat transfer equations, was used to determine these values [24].
Q=U×SA×(Tin−Tout)
(2)
where,
- Q: Heat lost or gained due to outside temperature (kJ·h−1)
- U: Overall heat transfer coefficient (kJ·h−1·m−2·°C−1)
- SA: Surface area of greenhouse (m2)
- Tin: Inside air set point temperature (°C)
- Tout: Outside air temperature (°C)
For the purpose of these estimations, 23.9 °C was used as the set point temperature of the greenhouse, as this is the optimal temperature for lettuce cultivation [5]. Polyethylene is one of the most common materials used in greenhouse construction, and the overall heat transfer coefficient for this material (24.5 kJ/h/m2/°C) was used in calculations [24]. The heating and air conditioning efficiency was conservatively estimated to be 75% and the ceilings were estimated to be 3.0 m high. The dimensions of the hypothetical greenhouse were estimated to be 63.7 m by 12.8 m. For comparison purposes, it was assumed that the hydroponic greenhouse would be located in Yuma, Arizona, the same location where the majority of non-hydroponic head lettuce in Arizona is produced [9]; therefore, the average monthly temperatures for this region were used to estimate the temperatures external to the greenhouse [25]. The final energy estimates were used in conjunction with the estimated yield values for hydroponic lettuce to create a metric in units of kJ/kg/y.
Together, the estimates of energy use related to supplemental artificial lighting, water pumps, and heating and cooling loads were combined to produce an overall estimate of energy use for the hydroponic production of head lettuce in Arizona in units of kJ/kg/y.
3. Results and Discussion
In terms of yield per area, the hydroponic production of lettuce in Arizona was found to be 11 ± 1.7 times greater than that of its conventional equivalent. Specifically, hydroponic lettuce production was calculated to result in a yield of 41 ± 6.1 kg/m2/y (±standard deviation, SD, here and in the following), while conventional lettuce production was projected to yield 3.9 ± 0.21 kg/m2/y ( Figure 1).
Figure 1. Modeled annual yield in kilograms per square meter of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Figure 1. Modeled annual yield in kilograms per square meter of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Water consumption between the hydroponic and conventional production of lettuce in Arizona was comparable on an area basis, but when normalized by yield the average was 13 ± 2.7 times less water demand in hydroponic production compared to conventional production. Specifically, hydroponic lettuce production had an estimated water demand of 20 ± 3.8 L/kg/y, while conventional lettuce production had an estimated water demand of 250 ± 25 L/kg/y ( Figure 2).
Results for energy consumption found that the hydroponic production of lettuce in Arizona requires 82 ± 11 more energy per kilogram produced than the conventional production of lettuce in Arizona. Dominating the hydroponic energy use are the heating and cooling loads at 74,000 ± 10,000 kJ/kg/y, followed by the energy used for the supplemental artificial lighting at 15,000 ± 2100 kJ/kg/y. The circulating pumps contributed the least to the total energy use at 640 ± 120 kJ/kg/y. In total, the hydroponic energy use was calculated to equal 90,000 ± 11,000 kJ/kg/y ( Figure 3).
The total energy use for the conventional production of lettuce in Arizona was calculated to be 1100 ± 75 kJ/kg/y ( Figure 3). This total was split between the energy use related to fuel usage at 330 ± 20 kJ/kg/y and groundwater pumping at 760 ± 74 kJ/kg/y.
Figure 2. Modeled annual water use in liters per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Figure 2. Modeled annual water use in liters per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods (Error bars indicate one standard deviation).
Figure 3. ( a) Modeled annual energy use in kilojoules per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods; ( b) The energy use breakdown related to the hydroponic production of lettuce; ( c) The energy use breakdown related to the conventional production of lettuce (Error bars indicate one standard deviation).
Figure 3. ( a) Modeled annual energy use in kilojoules per kilogram of lettuce grown in southwestern Arizona using hydroponic vs. conventional methods; ( b) The energy use breakdown related to the hydroponic production of lettuce; ( c) The energy use breakdown related to the conventional production of lettuce (Error bars indicate one standard deviation).
The objective of this study was to determine whether hydroponic lettuce production is a suitable and more sustainable alternative to conventional lettuce production in Arizona. According to the results presented, summarized in Table 1, while the hydroponic production of lettuce results in higher yields and more efficient water use, the controlled environment from which the hydroponic system produces its higher yields exhibits a higher energy demand.
Table 1. Summary of modeled annual data with standard deviations (S.D.).
| Production Method | Yield (kg/m2/y) | Water Use (L/kg/y) | Energy Use (kJ/kg/y) | | :-: | :-: | :-: | :-: | | Value | S.D. | Value | S.D. | Value | S.D. | | :-: | :-: | :-: | :-: | :-: | :-: | | Conventional | 3.9 | 0.21 | 250 | 25 | 1100 | 75 | | Hydroponics | 41 | 6.1 | 20 | 3.8 | 90,000 | 11,000 |
Table 1. Summary of modeled annual data with standard deviations (S.D.).
Higher yields of hydroponics result from the controlled environmental conditions maintained within the hydroponic greenhouse, which allow for continuous production year round. These conditions also promote a reduction in the number of days required for each harvest cycle, allowing for multiple crops per year. This benefit of hydroponic production is not unique to lettuce alone, but will vary depending on the operational parameters under which the crop is grown.
Similarly, most hydroponic systems will utilize water more efficiently than conventional farming. The volume of water consumed per plant in a hydroponic system is not different from that grown using conventional methods; however, the hydroponic system delivers the water more efficiently, with a larger percentage of the water going to plant evapotranspiration [26]. For example, lettuce has shallow roots, but is primarily irrigated through flood furrow irrigation in southwestern Arizona. Water not quickly absorbed by the roots is lost to percolation. Increases in the use of low-flow and more-targeted irrigation techniques could lower the overall water use of conventional farming.
As mentioned previously, most of the energy use for the hypothetical hydroponic greenhouse can be attributed to the heating and cooling loads. This is primarily due to the fact that the greenhouse was sited in Yuma, Arizona, an area which can have average temperatures of 34.7 °C in the summer and 14.1 °C in the winter [25]. For illustrative purposes, the percent heating and cooling energy demand by month is presented in Table 2. Greenhouses located in more moderate climates (i.e., climates closer to the greenhouse set point temperature) will experience a lower energy demand. In fact, in certain climates heating and cooling systems may not be required, but instead replaced by a passive ventilation system, thus reducing the overall energy demand considerably. The feasibility of hydroponic systems is heavily reliant on the climate of farming locations.
Table 2. Percent heating and cooling energy demand by month.
| Month | January | February | March | April | May | June | July | August | September | October | November | December | | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | | Average Temperature (°C) | 14.8 | 16.6 | 19.6 | 22.8 | 27.4 | 31.7 | 34.7 | 34.6 | 31.7 | 25.3 | 18.7 | 14.1 | | Percent Energy Demand | 12% | 8% | 6% | 1% | 5% | 10% | 14% | 14% | 10% | 2% | 6% | 13% |
Table 2. Percent heating and cooling energy demand by month.
The next highest use of energy for the hypothetical hydroponic greenhouse is for the supplemental artificial lighting, which is used to maximize crop yield and maintain consistent production year round [7]. Some systems use supplemental lighting to create a 24-h photoperiod, especially during the first few days of plant growth, whereas others may use supplemental lighting for only a few hours a day [5, 13]. In addition, small and low-output systems may not use artificial lighting at all. This study assumed that maximum yield was desired and did not perform a cost-benefit analysis of reducing or eliminating supplemental lighting. There are various studies that have tried to optimize supplemental lighting systems [14, 27, 28]. Progress in this area of research could lead to improved energy efficiency for hydroponic lettuce.
Due to the high energy demands, at this time, commercial hydroponics is not a suitable alternative to conventional lettuce production in Yuma, Arizona. mechanisms↗concepts↗One possible way to make commercial hydroponics a more sustainable and suitable alternative would be to relocate the greenhouse to an area where there are cheap and renewable sources of energy, such as solar, geothermal or wind power; though keeping in mind that the initial investment required for such technologies may be cost-prohibitive. For instance, a hydroponic greenhouse could be sited in an area without arable soil, or on previously developed areas with impervious pavement, where land is cheap and could be used for photovoltaic solar panels. mechanisms↗controlscontent_tagsAssuming a conversion efficiency of 14% and an average daily solar radiation value of 6.5 kWh per square meter per year [29] the ratio of greenhouse to solar panel area that would be required to offset the full energy demand of the hydroponic system is approximately 1:3.0, mechanisms↗with an understanding that the 24-h production cycle along with seasonal and day-to-day changes in solar radiation would require the system to be connected to the electrical grid. concepts↗This is still an improvement over conventional production, which uses ten times more land and water on a yield basis and requires arable land. Eliminating the need for arable land has other benefits including versatility in system siting and a potential reduction in the distance in which food must travel. limitationsPerforming a life cycle assessment that considers the environmental impact of food transportation could show whether this benefit of hydroponics is significant.
concepts↗concept_edgescontent_tags↗An alternative scenario to consider is one in which much of the energy-intensive characteristics of advanced and commercial hydroponic operations are abandoned in favor of simpler systems, a practice known as “simplified hydroponics” or “popular hydroponics.” mechanisms↗content_tagsThese operational changes come at the cost of total yield; however, such systems can still out-perform conventional systems by a factor of three to four on an area basis [30]. The Food and Agriculture Organization of the United Nations maintains that such systems can be as small as one square meter in size, but that most household simplified hydroponic gardens range between 10 to 20 square meters in size. Communities have also adopted simplified hydroponic gardens at sizes greater than 200 square meters—a scale in which it becomes viable to sell excess produce for income [30].
There are several limitations to this study that need to be considered. limitationsTo begin, the calculations for the conventional production of lettuce assumed that only one crop of lettuce can be grown in a given year. While this might be true, much of the land used to produce lettuce is dedicated to warm season crops when lettuce is not in production. It’s possible that these crops have water and energy requirements that differ from lettuce. limitationsAdditionally, the hydroponic greenhouse was assumed to be of commercial scale and was optimized for maximum production. sensesFurthermore, as seen in Equation (2), the greenhouse was assumed to be of a specific size and located in Yuma, Arizona (one of the most productive areas for lettuce grown conventionally in the United States). In reality, hydroponic greenhouses come in a variety of configurations and can be located almost anywhere. limitationsAs such, it would be reasonable to model multiple hypothetical greenhouses. Changing the assumptions surrounding the hypothetical greenhouses could produce alternative results. limitationssensesIn addition, this study solely focused on direct energy inputs to hydroponic and conventional lettuce production and did not consider energy embodied in chemical or material inputs. limitationsconcepts↗Performing a full life cycle assessment of hydroponics vs. conventional lettuce production could also produce alternative results, with labor hours potentially figuring prominently into the economic equation. limitationsLastly, this study did not discuss additional factors that might inhibit the successful implementation of hydroponics, such as energy scarcity and higher upfront capital costs.
4. Conclusions
Despite its high demand for energy, hydroponics remains a promising technology. Several factors could influence the feasibility of hydroponic production of crops, specifically lettuce, in the future. As more sophisticated control devices become available, the cost of maintaining the controlled environment of hydroponic greenhouses could decrease. The future availability of water, land, and food will also influence feasibility through increased demand. Increasing land and water scarcity will make the more land- and water-efficient hydroponic systems more appealing to city planners. Government and local grass-roots support could also influence the future of hydroponic farming, as subsidies could be used to offset the high initial cost of hydroponic infrastructure or more simplified hydroponic systems take hold.
claim_relationscontent_tagsAt this point in time, hydroponic farming of lettuce cannot be deemed a more sustainable alternative to conventional lettuce farming techniques, but it provides promising concepts that could lead to more sustainable food production. In summary, hydroponic gardening of lettuce uses land and water more efficiently than conventional farming and could become a strategy for sustainably feeding the world’s growing population, if the high energy consumption can be overcome through improved efficiency and/or cost-effective renewables.
Acknowledgments
The authors of this study would like to gratefully acknowledge Alizée Jenck for her time, guidance, and feedback.
Author Contributions
Guilherme Lages Barbosa, Francisca Daiane Almeida Gadelha, Natalya Kublik, Alan Proctor, Lucas Reichelm, Emily Weissinger, and Gregory M. Wohlleb contributed equally by performing the literature review, conducting the modeling analysis, and co-writing the manuscript. Rolf U. Halden directed the research and supervised the co-authors who completed this work in partial fulfillment of the requirements of the Fall 2014 Arizona State University course HON490-CEE470/570. All authors have given approval to the final version of the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
Relations on this page carry a quotation for the claim. Where a relation was separately evidenced — and so tested against the vocabulary for its type — the row says so; none of 2 relations here carry that mark. The rest are the tagger’s reading of the sentence quoted beside them.
| facet | tag | field | conf. | whose words |
|---|---|---|---|---|
| object_of_study | the land, water and energy required to grow lettuce hydroponically against conventionally, in Yuma, Arizona | object_of_study | the paper's words | |
| research_question | whether hydroponic lettuce production is a more sustainable alternative to conventional production on land, water and energy together | research_question | the paper's words | |
| contribution | the first quantitative land-water-energy comparison of conventional and hydroponic lettuce in the southwestern United States, identifying energy availability as the deciding factor | contribution | the paper's words | |
| claim_type | existence | claim_type | the paper's words | |
| phenomenon | agriculture already using 38.6% of ice-free land and 70% of withdrawn freshwater, against a growing population | phenomenon | the paper's words | |
| method | computational-model also in: computational-model | method | the paper's words | |
| mechanisms | mechanisms | mechanisms[0] | 0.95 | the tagger's words |
| mechanisms | mechanisms | mechanisms[1] | 0.95 | the tagger's words |
| mechanisms | mechanisms | mechanisms[2] | 0.95 | the tagger's words |
| mechanisms | mechanisms | mechanisms[3] | 0.95 | the tagger's words |
| mechanisms | mechanisms | mechanisms[4] | 0.95 | the tagger's words |
| limitations | conventional calculations assume one lettuce crop per year, while that land carries warm-season crops the rest of the year whose water and energy needs may differ | limitations[0] | 0.95 | the tagger's words |
| limitations | the hydroponic greenhouse was assumed to be commercial scale and optimised for maximum production - a best case | limitations[1] | 0.95 | the tagger's words |
| limitations | one fixed greenhouse size at one site (Yuma), while real greenhouses vary in configuration and location; the authors say modelling several would be reasonable and could produce alternative results | limitations[2] | 0.95 | the tagger's words |
| limitations | only DIRECT energy inputs were counted - energy embodied in chemical or material inputs was excluded | limitations[3] | 0.95 | the tagger's words |
| limitations | no full life cycle assessment was performed, and the authors say one could produce alternative results with labour hours figuring prominently | limitations[4] | 0.95 | the tagger's words |
| limitations | factors that could block implementation - energy scarcity and higher upfront capital costs - were not discussed | limitations[5] | 0.95 | the tagger's words |
| limitations | whether reduced food transport distance is a real benefit is left open, pending a life cycle assessment that considers transportation | limitations[6] | 0.95 | the tagger's words |
| controls | conventional figures were taken from crop budgets and government agricultural statistics rather than estimated, so one side of the comparison rests on observed production data eliminates: estimation error on the conventional side of the comparison - those figures come from crop budgets and government statistics rather than from the authors' model. It does NOT eliminate it on the hydroponic side, which is theoretical throughout · outcome: held | controls[0] | 0.95 | the tagger's words |
| controls | all four headline quantities are reported with a standard deviation - yield, water and energy for both systems - so the 11x and 82x ratios carry their own uncertainty eliminates: the 11x and 82x ratios being read as exact - every one of the four headline quantities carries a standard deviation · outcome: held | controls[1] | 0.95 | the tagger's words |
| controls | the solar offset needed to cover the hydroponic energy demand was computed from stated conversion efficiency and radiation values rather than asserted, and the grid dependence is stated with it eliminates: solar generation being asserted as a remedy without a cost - the greenhouse-to-panel area ratio is computed at about 1:3.0, and the grid dependence is stated with it, so the land saved is partly spent again · outcome: partial | controls[2] | 0.95 | the tagger's words |
| concepts | hydroponics also in: hydroponics | concepts[0] | 0.95 | the paper's words |
| concepts | conventional agriculture also in: conventional agriculture | concepts[1] | 0.95 | the paper's words |
| concepts | simplified hydroponics also in: simplified hydroponics | concepts[2] | 0.95 | the paper's words |
| concepts | life cycle assessment also in: life cycle assessment | concepts[3] | 0.95 | the paper's words |
| concepts | arable land also in: arable land | concepts[4] | 0.95 | the paper's words |
| concepts | crop budgets also in: crop budgets | concepts[5] | 0.95 | the paper's words |
| concepts | sustainability also in: sustainability | concepts[6] | 0.95 | the paper's words |
| concepts | greenhouse also in: greenhouse | concepts[7] | 0.95 | the paper's words |
| senses | the `comparison` - ONE SIDE IS MEASURED AND THE OTHER IS MODELLED, and the abstract says so in a clause easy to read past: conventional figures come from CROP BUDGETS AND GOVERNMENT STATISTICS, hydroponic figures are THEORETICAL DATA DERIVED BY ENGINEERING EQUATIONS POPULATED WITH LITERATURE VALUES. So `11x higher yields, 82x more energy` compares an observed system with a calculated one. Legitimate, and not what a reader assumes from the word `comparison` - every quantity on this record inherits it | senses[0] | 0.95 | the tagger's words |
| senses | the hydroponic greenhouse - A SINGLE OPTIMISED HYPOTHETICAL, not a survey of installations: commercial scale, optimised for maximum production, one fixed size, sited in Yuma. The authors say hydroponic greenhouses `come in a variety of configurations and can be located almost anywhere` and that modelling several would be reasonable. The figures are therefore a best case at one site | senses[1] | 0.95 | the tagger's words |
| senses | `energy` here is DIRECT ENERGY ONLY - the study excludes energy embodied in chemical or material inputs, and excludes labour. The 82x ratio is a ratio of direct inputs, and the authors say a full life cycle assessment could produce alternative results | senses[2] | 0.95 | the tagger's words |
| senses | `conventional agriculture` - DEFINED HISTORICALLY AND EXPLICITLY: growing crops in soil, in the open air, with irrigation and the active application of nutrients, pesticides and herbicides. A definition by PRACTICE rather than by yield or by certification, which is what makes the comparison well-posed | senses[3] | 0.95 | the tagger's words |
| claim_relations | critiques → the assumption that hydroponics is straightforwardly the more sustainable method - the authors conclude it cannot currently be deemed so for lettuce | claim_relations[0] | 0.95 | the tagger's words |
| concept_edges | hydroponics —contrasted-with→ conventional agriculture strength of the relation: 0.95 also in: hydroponics · conventional agriculture | concept_edges[0] | the tagger's words | |
| concept_edges | simplified hydroponics —is-a→ hydroponics strength of the relation: 0.9 also in: simplified hydroponics · hydroponics | concept_edges[1] | the tagger's words | |
| content_tags | hydroponics gave 11 ± 1.7 times higher yields and required 82 ± 11 times more energy than conventional lettuce production | content_tags.findings[0] | 0.95 | the tagger's words |
| content_tags | Hydroponic lettuce cannot yet be deemed a more sustainable alternative, though it offers promising concepts. | content_tags.findings[1] | 0.95 | the tagger's words |
| content_tags | energy availability is the deciding factor, making water-scarce regions with abundant renewables the attractive sites | content_tags.findings[2] | 0.95 | the tagger's words |
| content_tags | simplified hydroponics sacrifices yield but still out-performs conventional systems three- to four-fold on an area basis | content_tags.findings[3] | 0.95 | the tagger's words |
| content_tags | conventional agriculture also in: conventional agriculture | content_tags.definitions[0] | 0.85 | the tagger's words |
| content_tags | simplified hydroponics also in: simplified hydroponics | content_tags.definitions[1] | 0.95 | the tagger's words |
| content_tags | hydroponic: 41 ± 6.1 kg/m2/y yield, 20 ± 3.8 L/kg/y water, 90,000 ± 11,000 kJ/kg/y energy, per 815 m2 greenhouse unit — None | content_tags.quantities[0] | 0.95 | the tagger's words |
| content_tags | conventional: 3.9 ± 0.21 kg/m2/y yield, 250 ± 25 L/kg/y water, 1100 ± 75 kJ/kg/y energy — None | content_tags.quantities[1] | 0.95 | the tagger's words |
| content_tags | agriculture already uses 38.6% of ice-free land and 70% of withdrawn freshwater — None | content_tags.quantities[2] | 0.95 | the tagger's words |
| content_tags | greenhouse-to-solar-panel area ratio of about 1:3.0 to offset the full hydroponic energy demand, at 14% conversion efficiency — None | content_tags.quantities[3] | 0.95 | the tagger's words |
Tags that could not be placed in the text:
What was deliberately not tagged, and why, is recorded per paper by the tagger. Where a paper has no such record, this page says so rather than implying that nothing was declined.